PROJECT SUMMARY Telomere length (TL) has been established as a marker for cellular aging and as one of the hallmarks of aging. Studies have implicated age-related TL with a broad range of risk factors that predict disease morbidity and early mortality. Researchers interested in telomere science are nevertheless concerned about precision measures and reproducibility, which is a burning issue in the field and the sciences more broadly. A key question concerns the measurement of TL from different types of tissue cells for large population- based studies. Blood has commonly been used in TL studies. However, blood collection is relatively invasive, costly, and limiting in terms of where and by whom it can be collected. Scientific progress in the understanding of how TL affects health and aging will be greatly enhanced if researchers are not limited to the use of blood. How TL differ within- and between-individuals across common tissue cells and developmental stages using the T/S ratio, monochrome multiplex and absolute quantitative PCR (qPCR) methods is not known. Moreover, technical laboratory variations such as sample collection, storage conditions and pipetting techniques can also influence TL measurements. Results of this study may reveal an optimal tissue cell for large population-based research, clarify the precision of pipetting techniques and other technical laboratory procedures for studies on TL, and enable comparisons with TL results derived using different measurement methods. Aim 1 will determine, in 5 different cohorts from birth to age 80, the comparability of TL across commonly sampled tissues [whole blood/leukocytes, peripheral blood mononuclear cells (PBMCs), dried blood spots (DBS), cord blood, saliva, and buccal cells] using multiple qPCR assays. Aim 2 will examine technical laboratory variation on TL measurement by testing fresh vs. archived tissues and DNA, as well as various sample collection procedures and pipetting techniques (i.e., manual vs. automated robotic). Aim 3 will test association and estimated effect sizes of potential TL variation associated with different tissues and technical lab techniques involving multiple system-biology and phenotypic measures. This study is significant because its rigorous design will enable strong inferences regarding the impact of tissue comparability from birth to 80 years on TL, as well as technical laboratory variations and estimated effect sizes of potential TL variation using multiple qPCR methods. Combined findings from the telomere consortium will help generate consensus guidelines for TL measurement. This study is innovative because it will be the first to comprehensively assess six tissues from the same sample across developmental stages and lab techniques, and will lay the groundwork for future comparisons. The results and raw data generated by this proposal will be shared with the Telomere Research Network consortium to increase transparency, standardize measurement methods, and facilitate cross-method comparison studies.